High quality global journalism requires investment. Please share this article with others using the link below, do not cut & paste the article.

A group of businesses is preparing a legal challenge to prevent the government from beginning Brexit negotiations without an act of parliament.

The action, brought by law firm Mishcon de Reya, would potentially complicate Britain’s path to leaving the EU, given that a majority of MPs were in favour of remaining and many have continued to speak in favour of maintaining access to the single market.

High quality global journalism requires investment. Please share this article with others using the link below, do not cut & paste the article.

Candidates in the Conservative leadership election have so far treated the start of negotiations as within the next prime minister’s discretion.

Theresa May, favourite in the leadership debate, told ITV’s Peston on Sunday that she would only activate Article 50, the formal notification of Britain’s intention to leave, once the government’s negotiating stance was clear. That would mean the legal trigger would not occur this calendar year, a stance supported by the justice secretary Michael Gove. He said all EU citizens lawfully resident in the UK must retain their right of residence.

In contrast, two other candidates in the Conservative leadership election, Andrea Leadsom and Liam Fox, have said they would begin formal negotiations in September or shortly after. EU leaders have also said negotiations should begin once a new prime minister is in place.

Once Article 50 is activated, the UK would have to leave the EU within two years — unless all other member states consented to an extension.

Mischon de Reya’s legal challenge follows an article by three academics, Nick Barber, Tom Hickman and Jeff King. The article argued that the government would be violating parliamentary sovereignty if it activated Article 50 on its own — because it would render redundant rights established by the European Communities Act of 1972.

“In our constitution, parliament gets to make this decision, not the prime minister,” the academics said. In addition, an activation without a parliamentary vote would not be effective, because Article 50 of the Lisbon treaty requires a state to act “in accordance with its own constitutional requirements”.

High quality global journalism requires investment. Please share this article with others using the link below, do not cut & paste the article.

Kasra Nouroozi, a partner at Mishcon de Reya, said: “The result of the referendum is not in doubt, but we need a process that follows UK law to enact it. Article 50 simply cannot be invoked without a full debate and vote in parliament.”

The firm, which has not disclosed the names of its clients in the action, said that government lawyers argued that the executive did have the power to activate Article 50.

High quality global journalism requires investment. Please share this article with others using the link below, do not cut & paste the article.

In a sign of parliamentary activism, the Labour party’s Yvette Cooper called on Sunday for the government to guarantee the rights of EU citizens living in the UK, after Mrs May said they would be subject to negotiation.

Mrs May’s rival, Mr Gove, said that all EU citizens lawfully resident in the UK must retain their right of residence.

Last week three MPs — the Conservative Peter Bottomley, the SDLP’s Mark Durkan and the Greens’ Caroline Lucas — backed an early day motion, calling for the government not to invoke Article 50 “until the full proposals it intends to submit to the commission to activate the process of withdrawal from the EU are debated in full and voted upon by parliament.”

Despite its political ramifications, the referendum had no automatic effect in UK law. Leading members of the Conservatives and Labour have said the result should be respected, although the Liberal Democrats have said they will continue to fight for Britain to remain within the EU.